The present invention relates to nuclear fuel and in particular to a rack for the storage of nuclear fuel assemblies.
Fuel for commercial nuclear fission reactors typically includes a stack of enriched uranium dioxide pellets contained within a tube called a fuel rod. A fuel assembly includes about 200 to 300 fuel rods held in a square array by spacer grids and upper and lower end fittings.
It is common for nuclear fuel assemblies, when not in use in the reactor steam cycle, to be stored in pools of water at the nuclear reactor site or at another site. Fuel assemblies are stored both before and after they are used to produce energy in the reactor.
Typically, nuclear fuel assemblies are stored under water at reactor sites in some sort of rack structure, spaced from each other a sufficient distance to preclude a sustained nuclear fission chain reaction. One type of rack uses a pair of horizontal frames supporting the fuel assembly at the top and bottom in a vertical attitude. In another the fuel assemblies are inserted into cans. The top and bottom ends of the cans are supported by some form of horizontal frame. These types of structures have significant flexibility, a disadvantage in that it permits them to respond to seismic or other excitations raising the possibility of damage to the stored fuel assemblies and potential leakage of radioactive materials therefrom.
The lack of nuclear fuel reprocessing facilities has forced many utilities and fuel assembly manufacturers to increase the number of fuel assemblies stored in their storage pools. The most common way to increase capacity of a storage pool is to use a more closely spaced fuel assembly array. A sustained nuclear fission chain reaction is precluded in the closely spaced array by positioning a neutron absorbing material between the stored fuel assemblies. Current closely spaced fuel assembly rack designs require expensive fabrication techniques and inefficient use of structural material.
The present invention is a nuclear fuel assembly storage rack concept consisting of four identical plates joined in a novel arrangement to form a nuclear fuel assembly storage cell. A number of these storage cells can be joined in an array to form a modular rack structure of variable array size depending on the individual plant application and needs.
The storage cell is made of stainless steel plates which can be assembled on a mandrel to achieve required dimensional control and square corners on the inside of the cell. The plates can be joined together by either riveting or welding the easily accessible flange areas. Since the plates are identical, the advantages of part standardization are present. Material procurement, tooling, and fabrication are all simplified.
The shape of the plates and the arrangement of cells create a cavity between adjacent cell walls into which the neutron absorbing material can easily be placed.
The cells can be joined together to form a stiff array to permit the natural frequency of the structure to be made sufficiently high thus avoiding large seismic response. Seismic loads that are generated can be carried from cell to cell, rather than through an external framework, reducing the amount of structural steel needed. The rack structure can be designed to be free standing; ie: no braces between rack and pool walls thus eliminating structural steel bracing.
It is an object of the invention to provide a nuclear fuel assembly storage rack.
A further object of the present invention is a storage rack utilizing a single standardized structural part resulting in lower unit fabrication and reduced tooling costs.
Another object of the invention is a storage rack yielding the foregoing advantages and which is simple to fabricate, thereby reducing welds or riveting, simplifying dimensional controls and reducing inspections.
Another object of the invention is a storage rack yielding the foregoing advantages and which is modular in construction and thus may be easily adjusted to any storage pool configuration or plant arrangement.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate the preferred embodiment of the invention.